Naomi Watts is "very, very scared" about playing Marilyn Monroe in a new biopic

The actress - who recently became attached to Clint Eastwood's forthcoming movie 'J. Edgar' in the role as his secretary Helen Gandy - is set to play the iconic screen siren in an adaptation of Andrew Dominik's 'Blonde' and she admits the nerves about the part are getting to her.

She said: "It's a huge pressure. We all feel we know her. I'm very, very scared about that."

It is not the only movie role she is nervous about - she also felt "daunted" to be playing real life CIA agent Valerie Plame - whose cover was busted by the White House when her husband wrote an anti-Bush editorial piece - in 'Fair Game'.

Asked about her feelings she said: "Daunting. I'm a lot more fragile than she is. To me, she seems almost invincible."

Naomi is not the only star to be portraying Marilyn in a movie about her life - Michelle Williams is also set to play her in 'My Week With Marilyn', and she too admits to feeling overawed by the project.

She previously said: "I feel like we live together. At a certain point something else does take over. I don't quite feel myself these days."